Yarn spinning processes and suitable apparatus are known from U.S. Pat. No. 4,130,983 for the so-called "open-end spinning" of individual fibers into a twisted thread or the similar "round-about spinning" to cover a continuous core thread with a sheath of individual fibers, these processes being carried out by twisting together the individual fibers on a defined thread formation line in the zone of the gap between two surfaces moving in opposite directions and by taking off the thread perpendicularly or obliquely to the direction of travel of the moving surfaces. In the apparatus for such spinning processes, it is possible to use a variety of cooperating elements as the moving surfaces, but as described in said U.S. Pat. No. 4,130,983 and also in our copending application Ser. No. 7,944, filed Jan. 20, 1979, these moving surfaces are preferably designed as cylindrical, hyperbolic or conical rollers or drums, particularly as air-permeable, perforated rollers or sieve drums which rotate in the same direction and form between a pair of them a narrow gap or nip as a thread forming zone. The narrowest gap between the two drums or rollers is essentially defined by parallel mantle lines lying in substantially one common plane.
In our application Ser. No. 7,944, it is suggested that the width of the narrowest gap can be narrowed in the draw-off direction of the thread in order to assist the axial conveying movement of the thread being formed. This proposed adjustment of the gap was based on the theory that a torsional movement is imparted to the thread being formed at a position where it is already present as a finished structure, as also demonstrated for example in the German published application No. 2,518,754. Other devices such as disc members at the outlet end or draw-off end of the drums or rollers were also suggested as an alternative means of assisting the axial conveying movement of the thread.